1) Field of the Invention
The invention is concerned with a vaccine composition for the protection of pigs against Actinobacillus pleuropneumoniae (App) infection and also with a method for protecting pigs by administering such a vaccine.
2) Description of the Prior Art
Porcine pleuropneumonia, a major respiratory disease in pigs, is spread out world-wide and causes severe economic losses to the pig industry due to peracute deaths, treatment of acutely sick pigs and the delays in marketing of chronically infected animals. As the etiological agent of this disease Actinobacillus pleuropneumoniae (A. pleuropneumoniae) has been identified. It is transmitted primarily by direct contact between animals, and the resulting infection produces a clinal course varying from peracute to chronic. The disease is primarily an infection of the respiratory tract having the clinical signs of high fever, severe respiratory distress, coughing and anorexia. The onset of the disease is rapid and morbidity and mortality are high. Pathologically of interest are the development and distribution of pneumonic lesions in the lungs.
Naturally, it has been attempted to control such A. pleuropneumoniae infections among pigs by vaccination programs.
To this end pigs have been vaccinated with bacterins, inactivated A. pleuropneumoniae bacteria. A disadvantage of such a vaccine is the concomitant serious side reactions. Furthermore, bacterin vaccination results primarily in antibodies elicited against (lipo)polysaccharides which are only specific for a certain serotype of A. pleuropneumoniae and hence are not protective against other A. pleuropneumoniae serotypes. In addition, bacterins of A. pleuropneumoniae only elicit a minor protection against field infection.
Capsule extracts of A. pleuropneumoniae have also been reported as protective antigens. However, immunization of pigs and mice with such extracts provided only partial immunity. In addition, capsule based vaccines only induce homologous protection, i.e. pigs vaccinated with a capsule vaccine derived from A. pleuropneumoniae of a specific serotype are not protected against challenge with A. pleuropneumoniae of a different serotype.
Live attenuated A. pleuropneumoniae vaccines also suffer from a number of drawbacks including the risk of inoculating animals with inadequately attenuated pathogens and the possibility that the attenuated bacteria may revert to a pathogenic state resulting in disease of the inoculated animals and the possible spread of the pathogens to other animals.
Hence, there is a long felt need for an A. pleuropneumoniae vaccine which is safe, serotype independent and induces a strong protective immune response in pigs.